Method of making an electrophoretic display panel with interleaved local anode

ABSTRACT

A method of manufacturing an improved an electrophoretic display having a cathode/grid/local anode matrix and a remote anode includes forming the local anode lines in the same plane as the grid lines from the same material and in the same fabricating step. The local anode lines are insulated from the grid lines and are interleaved therewith, each being formed on a common layer of photoresist. It is preferred that each grid line be associated with one local anode line, that the grid lines have tines and that the local anode lines be disposed between the tines.

This is a division of application Ser. No. 07/746,854, field Aug. 19,1991 now U.S. Pat. No. 5,216,416.

FIELD OF THE INVENTION

The present invention relates to an electrophoretic display panelapparatus and methods for making same and, more particularly, toelectrophoretic display panels with a local anode having elements whichare interleaved with the grid elements of the display for assisting inthe control of pigment particle migration and position.

DESCRIPTION OF THE PRIOR ART

Electrophoretic displays (EPIDS) are now well known. A variety ofdisplay types and features are taught in several patents issued in thenames of the inventors herein, Frank J. DiSento and Denis A. Krusos andassigned to the assignee herein, Copytele, Inc. of Huntington Station,N.Y. For example, U.S. Pat. Nos. 4,655,897 and 4,732,830, each entitledELECTROPHORETIC DISPLAY PANELS AND ASSOCIATED METHODS describe the basicoperation and construction of an electrophoretic display. U.S. Pat. No.4,742,345, entitled ELECTROPHORETIC DISPLAY PANELS AND METHODS THEREFOR,describes a display having improved alignment and contrast. Many otherpatents regarding such displays are also assigned to Copytele, Inc. Onepending patent application which may have some relevance to the presentinvention is U.S. Pat. No. 5,053,763 entitled DUAL ANODE FLAT PANELELECTROPHORETIC DISPLAY APPARATUS, each of which shall be describedbelow.

The display panels shown in the above-mentioned patents operate upon thesame basic principle, viz., if a suspension of electrically chargedpigment particles in a dielectric fluid is subjected to an appliedelectrostatic field, the pigment particles will migrate through thefluid in response to the electrostatic field. Given a substantiallyhomogeneous suspension of particles having a pigment color differentfrom that of the dielectric fluid, if the applied electrostatic field islocalized it will cause a visually observable localized pigment particlemigration. The localized pigment particle migration results either in alocalized area of concentration or rarefaction of particles dependingupon the sign and direction of the electrostatic field and the charge onthe pigment particles. The electrophoretic display apparatus taught inthe foregoing U.S. Patents are "triode-type" displays having a pluralityof independent, parallel, cathode row conductor elements or "lines"deposited in the horizontal on one surface of a glass viewing screen. Alayer of insulating photoresist material deposited over the cathodeelements and photoetched down to the cathode elements to yield aplurality of insulator strips positioned at right angles to the cathodeelements, forms the substrate for a plurality of independent, parallelcolumn or grid conductor elements or "lines" running in the verticaldirection. A glass cap member forms a fluid-tight seal with the viewingwindow along the cap's peripheral edge for containing the fluidsuspension and also acts as a substrate for an anode plate deposited onthe interior flat surface of the cap. When the cap is in place, theanode surface is in spaced parallel relation to both the cathodeelements and the grid elements. Given a specific particulate suspension,the sign of the electrostatic charge which will attract and repel thepigment particles will be known. The cathode element voltage, the anodevoltage, and the grid element voltage can then be ascertained such thatwhen a particular voltage is applied to the cathode and another voltageis applied to the grid, the area proximate their intersection willassume a net charge sufficient to attract or repel pigment particles insuspension in the dielectric fluid. Since numerous cathode and gridlines are employed, there are numerous discrete intersection pointswhich can be controlled by varying the voltage on the cathode and gridelements to cause localized visible regions of pigment concentration andrarefaction. Essentially then, the operating voltages on both cathodeand grid must be able to assume at least two states corresponding to alogical one and a logical zero. Logical one for the cathode may eithercorrespond to attraction or repulsion of pigment. Typically, the cathodeand grid voltages are selected such that only when both are a logicalone at a particular intersection point, will a sufficient electrostaticfield be present at the intersection relative to the anode to cause thewriting of a visual bit of information on the display through migrationof pigment particles. The bit may be erased, e.g., upon a reversal ofpolarity and a logical zero-zero state occurring at the intersectioncoordinated with an erase voltage gradient between anode and cathode. Inthis manner, digitized data can be displayed on the electrophoreticdisplay.

An alternative EPID construction is described in U.S. Pat. No.5,053,763, referred to above, which relates to an electrophoreticdisplay in which the cathode/grid matrix as is found in triode-typedisplays is overlayed by a plurality of independent separatelyaddressable "local" anode lines. The local anode lines are depositedupon and align with the grid lines and are insulated therefrom byinterstitial lines of photoresist. The local anode lines are in additionto the "remote" anode, which is the layer deposited upon the anodefaceplate or cap as in triode displays. The dual anode structureaforesaid provides enhanced operation by eliminating unwanted variationsin display brightness between frames, increasing the speed of thedisplay and decreasing the anode voltage required during Write and Holdcycles, all as explained in U.S. Pat. No. 5,053,763, which isincorporated herein by reference.

An examination of U.S. Pat. No. 5,053,763 reveals that the local anodestructure employed therein is realized by applying a layer ofphotoresist over the grid elements, which are formed from a first metal,such as, chrome. A layer of a second metal, e.g., nickel or aluminum, isapplied over the photoresist layer. Yet another layer of photoresist isapplied over the second metal layer, and is then masked, exposed anddeveloped in the same form as the grid elements. The second metal layeris then etched with a suitable solution. The photoresist between thefirst and second metal layers is then plasma etched. A layer of SiO₂ isthen deposited over the resulting structure.

It is an object of the present invention to provide an alternativestructure and method for making the remote anode/cathode/grid matrixthan that shown in U.S. Pat. No. 5,053,763.

SUMMARY OF THE INVENTION

The problems and disadvantages associated with conventionalelectrophoretic displays are overcome by the present invention whichincludes in an electrophoretic display of the type having: a cathodematrix comprising a plurality of parallel lines arranged in a givendirection, a grid matrix insulated from the cathode matrix andcomprising a plurality of parallel lines each perpendicular to thecathode lines to form an X-Y addressing matrix, and a conventional anodeelectrode separated from the X-Y matrix, the space between the anodeelectrode and the X-Y matrix accommodating an electrophoretic dispersionincluding pigment particles suspended in a fluid; the improvementtherewith of an additional anode electrode comprising a plurality ofparallel lines each associated with and insulated from the grid lines.The additional anode electrode is disposed within a plane shared by thegrid matrix and operates to control the path of the pigment particles toand from the X-Y matrix and to allow excess pigment to remain at theconventional anode electrode.

BRIEF DESCRIPTION OF THE FIGURES

For a better understanding of the present invention, reference is madeto the following detailed description of an exemplary embodimentconsidered in conjunction with the accompanying drawings, in which:

FIG. 1 is an exploded perspective view of an electrophoretic display inaccordance with an exemplary embodiment of the present invention.

FIG. 2 is an enlarged plan view of a selected local anode elementinterleaved with a selected grid element in accordance with an exemplaryembodiment of the present invention and as shown in FIG. 1.

FIG. 3 is a cross-sectional view of the electrophoretic display shown inFIG. 1 in the unexploded state, taken along section line III--III andlooking in the direction of the arrows.

FIG. 4 is an enlarged plan view of a selected grid and/or local anodeelement structure as is taught in prior U.S. Pat. No. 5,053,763 filed bythe inventors herein.

FIG. 5 is a cross-sectional fragmentary view of an electrophoreticdisplay in accordance with U.S. Pat. No. 5,053,763 and whichincorporates the element structure shown in FIG. 4.

DETAILED DESCRIPTION OF THE FIGURES

FIG. 1 shows an electrophoretic display 10 in accordance with thepresent invention. The display 10 has an anode faceplate 12 and acathode faceplate 14 which are sealably affixed on either side of aninterstitial spacer 16 to form a fluid-tight envelope for containing adielectric/pigment particle suspension or electrophoretic fluid (notshown) . The faceplates 12 and 14 are typically flat glass plates uponwhich are deposited conductor elements to comprise the situs ofelectrostatic charge for inducing motion in the electrophoretic fluid.The techniques, materials and dimensions used to form the conductorelements upon the faceplates and the methods for making EPIDS, ingeneral, are shown in U.S. Pat. Nos. 4,655,897, 4,732,830 and 4,742,345which patents are incorporated herein by reference.

In the invention, as depicted in FIG. 1, for example, a plurality ofindependent, electrically conductive cathode lines 18, shown here ashorizontal rows, are deposited upon the cathode faceplate 14 usingconventional deposition and etching techniques. Of course, theorientation of the cathode lines depends upon the orientation of thescreen, which, if rotated 90 degrees, would position the cathode linesvertically, thus, the cathode lines are arbitrarily defined ashorizontal. It is preferred that the cathode elements 18 be composed ofIndium Tin Oxide (ITO) as set forth in U.S. Pat. No. 4,742,345. Aplurality of independent grid conductor lines 20 are superimposed in thevertical over the cathode elements 18, i.e., at right angles thereto,and are insulated therefrom by an interstitial photoresist layer 22 (seeFIG. 3). The grid elements 20 may be formed by coating the photoresistlayer 22 with a metal, such as nickel, using sputtering techniques orthe like, and then selectively masking and etching to yield theintersecting but insulated configuration shown in FIG. 1. Each cathodeand grid element 18, 20 terminates at one end in a contact pad 24c and24g, respectively, or is otherwise adapted to permit connection todisplay driver circuitry (not shown). An anode 26 is formed on aninterior surface of the anode faceplate 12 by plating with a thin layerof conductor material, such as, chrome.

Whereas the foregoing components have been previously described in priorpatents and applications of the present Applicants, the presentinvention includes a novel local anode 28 structure. As stated above,the benefits and operation of an EPID having a local anode have beenrecognized and described in U.S. Pat. No. 5,053,763 by the presentApplicants. Previously, however, the local anode lines have been formedsuperimposed over and in alignment with the grid elements, and separatedtherefrom by an interstitial layer of photoresist insulation (see FIG.5). In the present invention, the local anode 28 lines are formed at thesame time, of the same material and in the same plane as the gridelements 20. This is accomplished by interleaving the local anode 28 andgrid 20 elements. Thus, the mask that was used to form the plurality ofgrid lines has been altered such that a plurality of grid lines and aplurality of local anode lines are simultaneously formed by a singlemask. After formation, a SiO₂ coating can be applied over the grid/localanode/cathode complex as set forth in U.S. Pat. No. 5,053,763. Thedisplay is also operated in the same fashion as in that application.

FIG. 2 shows an exemplary configuration for a single grid line 20, asingle local anode line 28 and their interleaving. As has beenrecognized previously, the configuration of the grid lines as a tinedelement, i.e., a element having a plurality of coextensive parallelforks 30 emanating from a common area, here the grid contact pad 24g,improves display brightness as described in U.S. Pat. No. 4,742,345. Inthe embodiment shown in FIG. 2, the local anode 28 is depicted as havinga single elongated portion 32 emanating from a contact pad portion 241a.The elongated portion 32 of the local anode 28 extends between the forks30 of the grid line 20, and, in this sense, interleaves with the gridline 20. It should be noted that the local anode 28 could also beprovided with forks like those of the grid line 20, and in that event,the interleaving could be in the form of alternating grid and localanode forks. Indeed, any number of grid forks 30 (elongated portions)and local anode forks or elongated portions 32 could be employed. It isrequired, however, that they be insulated one from another, and, inorder to provide a regular coordinate grid along with the cathode lines18, should be substantially parallel to each other and perpendicular tothe cathode lines 18. It is preferred that the local anode line 28 asshown in FIG. 2 have a width of approximately 30 microns, that a spacingof 12 microns separate the elongated portion 32 of the local anode 28from the forks 30 of the grid line 20, and that the grid forks 30 beapproximately 10 microns wide with an interfork spacing of 12 microns.These dimensions provide a local anode 28 which is wider than the gridforks 30 and which allows better pigment hiding than if the local anodewere narrower. Overall, the interleaved grid and local anode elementsconfigured according to these dimensions have an open area to closedarea ratio of approximately 40%, which is within the range of normaltriode EPIDS and a screen produced in accordance with these dimensionshas a normal display brightness. Open area ratio should be in the rangeof 30% to 60% for adequate screen brightness.

To form an EPID 10 like that shown in FIG. 1, the parts may be assembledin a stack and placed in an oven for baking. The spacer 16, in thatcase, would be coated on surfaces which contact adjacent elements with amaterial which would become plastic at baking temperatures, such as,epoxy. Upon baking, the meltable material flows and the elements form alaminate upon cooling. Of course, other methods exist within the scopeof the normally skilled artisan for assembling the elements of the EPID10 shown, such as, e.g., gluing. The lamination of the EPID elementsforms an envelope for containing the dielectric fluid/pigment particlesuspension.

FIG. 3 shows the electrophoretic display of FIG. 1 utilizing theinterleaving configuration shown in FIG. 2 assembled and incross-section. The anode 26 in the embodiment shown, is a plate-likearea of conductor material having a length and width essentiallymatching that of the cathode/grid/local anode matrix, i.e., coextensivewith the matrix, as is taught in the above referenced patents andapplications of the present Applicant. Unlike previous teachings, thepresent invention has the local anode 28 elements deposited uponphotoresist layer 22 in the same plane and by the same manufacturingstep as the grid elements 20 (the individual forks 30 being shown incross-section). Since all conductor elements are quite thin, they extendbeneath the interstitial spacer 16 without special provision and atleast one end thereof provides a terminal exterior to the envelope forconnecting display driver circuitry (not shown).

The proportions of the grid and local anode lines as shown in FIGS. 1-3have been distorted for the purposes of illustration, viz., theelongated portions would be long enough to extend substantially theentire height of the cathode faceplate 14, whereas the width of theindividual lines would be small enough to accommodate in the order of1,700 lines on an 8.5"×11" screen.; Thus, in real displays the grid andanode lines are very thin and elongated. A workable panel would have alarge number of intersections, e.g., 2,200×1,700 or a total of 3,740,000separately addressable intersection points. For ease of illustration,only a few cathode lines 18, grid lines 20, and local anode lines 28 aredepicted. More illustrations of electrophoretic displays, theircomponents and electrical circuitry can be seen by referring to U.S.Pat. Nos. 4,742,345 and 4,772,820, each being awarded to the inventorsherein and which are incorporated by reference herein.

FIGS. 4 and 5, are illustrations of certain features of EPIDS disclosedby the Applicants herein in U.S. Pat. No. 5,053,763 and are included forthe purpose of providing a comparison to the present invention. Elementshaving essentially the same form and function as corresponding elementsin the present invention are labelled with the same reference numerals.Common elements in the prior EPIDS which have been altered in thepresent invention are flagged by the suffix "pa". FIG. 4 illustrates theconfiguration for a tined grid (and local anode) element 20pa previouslydisclosed in U.S. Pat. No. 5,053,763. On comparison to the grid element20 configuration taught by the present invention, it should be observedthat, while the tined configuration is retained, a spacing must beprovided centrally to accommodate the interleaved anode line.

FIG. 5 illustrates the stacking of the local anode elements 28pa uponthe grid elements 20pa previously used by the applicants in EPIDS havinga remote and a local anode. It should be appreciated that this stackingconfiguration is done in several steps and that the local anode 28pamust be closely aligned with the grid elements 20pa for effectiveoperation. The present invention has neither of these requirements.Another difference between the present invention and that shown in FIG.5 is that the local anode 28 of the present invention alters thedistribution of pigment particles in the plane of the grid and the localanode. In contrast, in the device shown in FIG. 5, the local anodeeffects pigment concentration at the grid by drawing it into a planeremoved from the grid.

It should be understood that the embodiments described herein are merelyexemplary and that a person skilled in the art may make many variationsand modifications without departing from the spirit and scope of theinvention as defined in the appended claims.

We claim:
 1. A method for fabricating the cathode/grid/local anodematrix on the cathode faceplate of a dual anode electrophoretic displaycomprises performing, in substantially the order shown, the steps of:(a)forming a plurality of cathode lines on said cathode faceplate; (b)depositing a first layer of photoresist over said cathode lines; (c)coating said first layer of photoresist with a conductor material; (d)coating said conductor material with a second layer of photoresist; (e)masking said second layer of photoresist with a mask corresponding tothe shape of a plurality of grid lines interleaved with a plurality oflocal anode lines; (f) exposing said second layer of photoresist throughsaid mask; (g) developing said second layer of photoresist (h) acidetching said conductor material coating where not covered by said secondlayer of photoresist remaining after developing; and (i) plasma etchingsaid first and said second layers of photoresist where not covered bysaid conductor material remaining after said step of acid etching. 2.The method of claim 1, further including the step of applying a SiO₂coating over said cathode/grid/local anode matrix after said step ofplasma etching.
 3. The method of claim 2, wherein said step of coatingwith a conductor material includes sputtering a layer of chrome uponsaid first layer of photoresist.
 4. The method of claim 2, wherein saidstep of coating with a conductor material includes sputtering a layer ofaluminum upon said first layer of photoresist.
 5. A method formanufacturing a dual anode electrophoretic display comprising the stepsof:providing a cathode faceplate having a plurality of parallel,electrically conductive cathode lines formed thereon; coating saidcathode liens at selected intervals with a layer of insulating material;simultaneously forming from a single layer of conductive material aplurality of interleaved electrically conductive grid lines and localanode lines over said cathode liens such that said grid lines and saidlocal anode lines are insulated form each other and from said cathodelines.
 6. The method of claim 5, wherein said forming stepcomprises:coating said layer of insulating material with a conductormaterial; coating said layer of conductor material with a second layerof insulating material; and selectively etching said conductive layerand said second layer to define said interleaved grid and local anodelines.
 7. The method of claim 6, wherein said first and secondinsulating layers comprise photoresistive material.
 8. The method ofclaim 6, further including masking said second layer with a maskcorresponding to the shape of said grid lines interleaved with saidlocal anode liens prior to said etching step.
 9. The method of claim 8,further comprising developing said second layer through said mask. 10.The method of claim 9, wherein said etching step comprises acid etchingareas of said conductive layer not covered by said second layer aftersaid developing step.
 11. The method of claim 10, wherein said etchingstep further comprises plasma etching areas of said first and secondlayers of insulating material not covered by conductive material aftersaid step of acid etching.
 12. The method of claim 5, further includingthe step of applying a coating of silicon dioxide over said interleavedgrid and local anode lines.
 13. The method of claim 5, wherein said gridliens and said local anode lines are arranged at right angles relativeto said cathode lines.